Self-recuperative burners

ABSTRACT

A furnace has a self-recuperative burner mounted in a port in a wall of the furnace so as to form a products passage to conduct products of combustion from the furnace chamber to outside the furnace. The burner includes an air passage which leads to a nozzle mounted in the inlet of a combustion chamber and which is in heat-exchange relationship with the products passage. The air passage extends at first forwards beyond the inlet of the combustion chamber and then rearwards to the nozzle so that the air-heating surface area is more effective and metal in the hottest part of the products passage is cooled.

United States Patent 1i 3,695,816 Oeppen et al. [4511 Oct. 3, 1972 [5SELF-RECUPERATIVE BURNERS FOREIGN PATENTS OR APPLICATIONS T [72]Inventors: Brian pp J n m n n; 114,844 7/1926 Switzerland ..166

Stanley Sourbutts, all of War- WIckshlre, England PrimaryExaminer-Edward G. Favors [73] Assignee: The Gas Council, London,England Atmmey Kemn Palmer Estabrook [22] Filed: Oct. 30, 1970 57 STR C[21] Appl- No; 85,619 A furnace has a self-recuperative burner mountedin a port in a wall of the furnace so as to form a products 30 F i li ip i Data passage to conduct products of combustion from the N l 19 9furnace chamber to outside the furnace. The burner 6 Great Bmam"53674/69 includes an air passage which leads to a nozzle mounted in theinlet of a combustion chamber and [2%] (g! "Ml/16%;; which is in heatexchange relationship with the [58] Fri id 161 products passage. The airpassage extends at first for- 1 0 re wards beyond the inlet of thecombustion chamber 56] R f r Cted and then rearwards to the nozzle sothat the air-heate e ences l ing surface area is more effective andmetal in the hot- UNITED STATES PATENTS test part of the productspassage is cooled. 3,101,773 8/1963 Buaha ..43l/166 5 Claims, 3 DrawingFigures 1 SELF-RECUPERATIVE BURNERS a products passage for conductingproducts of combustion to the exterior of the fumace,'and the combustionair passage of the bumer being arranged to be disposed in use inheat-exchange relationship with the space to constitute a recuperator inwhich products of combustion preheat the combustion air passing alongthe air passage. Such self-recuperative burners are hereinafter referredto as being of the kind described."

Such a burner may be arranged to fire directly into a furnace chamber,in which case the furnace chamber together with the products passageconstitute the space. Alternatively a burner may heat the furnacechamber indirectly by being arranged to fire into a radiant tube inwhich case it is the interior of the radiant tube which together withthe products passage constitute the space.

For direct firing a humor is normallyarranged in use to project into ahole provided in a wall of the furnace, the products passage beingformed by a gap between the burner and the internal surface of the hole.In the case of a burner firing into a radiant tube-the burner isnormally enclosed in a tube between which tube and the burner a productspassage is defined.

Hitherto in self-recuperative burners of l the kind described thecombustion air passage does not extend beyond the inlet end of thecombustion chamber, thus what may be a relatively largeproportion of thelength of a burner is not used for heat transfer. In such burners metalparts, such as members supporting refractory components, for example,the combustion chamber, extend or are disposed beyond the inlet end ofthe combustion chamber and are exposed to temperatures at or approachingthose in the furnace chamber. Hitherto to avoid rapid deterioration ofsuch metal parts burners have been confined to use in applications wherethe products of combustion have temperatures less than about l,l00 C.

Normally, in use, burners are disposed so that their combustion chambersare disposed within the thickness of the furnace wall with their outletsat the inner surface of the wall for direct firing or in a correspondingposition in the case of indirect firing. The products of combustionduring their flow along a products passage lose heat to the furnacewall. Therefore, where known burners are in use, as there is norecuperator beyond the inlet end of the combustion chamber and theproducts of combustion have already lost heat to the furnace wall beforereaching the recuperator, the recuperator is relatively inefficient.

The present invention consists in a self-recuperative burner of the kinddescribed wherein the combustion air passage extends, in heat-exchangerelationship with the space, forwards, with respect to the direction offlow in the combustion chamber, beyond the inlet end of the combustionchamber before turning back to the nozzle.

The combustion chamber of such a burner may be of circular cross-sectionin which case tubes may be mounted one within another to define annularcrosssection air passages and the burner may be mounted in acylindricalport in a furnace wall to define an annular cross-sectionproducts passage so that a recuperator so formed surrounds the burner.However, in the case of a burner with a narrow rectangularcross-sectional combustion chamber two recuperators may be provided oneadjacent each of the longer sides of the combustion chamber.

Where the normal layout of such burners is followed, that is to say,with the combustion chamber within the thickness of the furnace wall andwith the outlet of the a combustion chamber at the inner surface of thefurnace wall, the air passage may extend to or nearly to the inlet endof the products passage before turning back to the nozzle. BY suitabledesign and with appropriate dimensions metal parts beyond the inlet endof the combustion chamber can now be cooled by the recuperator so thatthey can be kept below a temperature of l, 1 00 (I. even though theproducts of combustion are greatly above that temperature. This extendsthe range of applications for self-recuperative burners intohighertemperature heat treatment processes, with the advantage that itis in such applications that a degree of air preheating whichcontributes markedly to thermal efficiency is most readily obtained.

If, in a high-temperature application, a self-recuperative burner isdesigned so that at maximum throughput, the maximum temperature of themetal recuperator is, say 1,000C, a problem can arise when the burner isturned to low-fire. The recuperator (as with most heat exchangers) ismore efficient at low through-puts, and the combustion air and metaltemperatures will increase compared with their values at high-fire. Themetal temperature could therefore exceed the maximum allowable for anacceptable life. One way of overcoming this is to design a lessefficient recuperator. The combustion air pre-heat temperature is less,and the maximum metal temperature, even at low flows, can be held below1,000C. This method, however, is obviously wasteful, in that thecombustion air pre-heat becomes less at high-fire'than that which couldsafely be obtained.

A better way of overcoming this problem according to afurther feature ofthe present invention is to design the recuperator optimally to meet theoperating conditions at high-fire and to provide: means for bleedingfrom the air passage just before the nozzle air in excess of combustionrequirements at low-fire. In this way sufficient flow of air can bemaintained through the recuperator to prevent an excessive rise in metaltemperature.

In addition to such high-temperature applications, the invention hasadvantages in the field or lower-temperature heating, since with it aself-recuperative burner becomes more economical, in view of theincreased heat transfer area made available by the provision of theextended air passage.

Throughout the range of temperatures, the extended air passage reducesthe flow of heat from the combustion chamber, and from the hot gasesflowing outwards along the products passage, to the furnace wall. Thisis because some heat istransferred from the combustion chamber and fromthe products passage to the ingoing combustion air flowing in theextended air passage, instead of to the furnace wall.

The invention is advantageous when it is a question of replacing a towngas burner by a burner for use with natural gas. The latter needs tohave a longer combustion chamber, provision of .which in knownselfrecuperative burners of the kind described would result in either areduction in the heat exchanger length or an increase in the overalllength of the burner. As the full thickness of the furnace wall can beused for heat recovery with the present invention, this disadvantage ofburning natural gas is avoided.

Embodiments of the invention will now be described by way of exampleonly, with reference to the accompanying drawings, in which:

FIG. 1 is a diagrammatic longitudinal sectional view of a burneraccording to the invention shown as mounted in use in part of a furnacewall, I

FIG. 2 is a fragmentary longitudinal view of the burner shown in FIG. 1,and

FIG. 3 is a view similar to FIG. 2 of a different burner according tothe invention.

The burner 1 shown in FIGS. 1 and 2 is mounted in a cylindrical portformed in a wall 2 of a furnace. An annular cross-section productspassage is formed between the burner 1 and the wall of the port, and hasan outlet 4. Fuel in the form of gas is supplied to the burner 1 throughan inlet 5 and air is supplied to the burner through an inlet 6.

In FIG. 2 a nozzle 9 closes the inlet end of a combustion chamber 10 andis supplied with gas through a central tube 11. A combustion air passage12 is formed by the annular space between concentric tubes 13 and 14.The forward end of the outer tube 13 is joined by an end wall 15 (curvedin the present instance) to an inner concentric tube 16 inside the tube14. The rear end of the tube 16 is joined to an annular plate 17surrounding the nozzle 9. The forward end of the tube 14 is spaced fromthe end wall 15 to allow air to flow round it and then back along theannular space between the tube 14 and the tube 16 into an air box 18 therear wall of which is an annular plate 19 which is welded to the centraltube 11 and extends radially outwards to the tube 14. The central tube11 is covered with insulation 20. Refractory material 21 surrounds thecombustion chamber 10 and extends radially at the front to the outerdiameter of the tube 13.

The tube 13 being the outermost tube forms with the wall of the port theannular products passage 3. The front of the refractory material 21 iscoincident with the inside surface of the furnace wall 2. The burner 1having the end wall 15 spaced somewhat from the inside surface of thefurnace wall 2 by the refractory material 21, is particularly suitablefor operation when the products of combustion reach very hightemperatures. For operation at lower temperatures this spacing is not sonecessary and the end wall 15 could be flush with the inner surface ofthe furnace wall.

When the burner l is in operation products of combustion leave thefurnace along the products passage 3. Heat is radiated by the outer tube13 to the tube 14 and from the tube 14 to the tube 16. In this way, inthe region of the combustion chamber 10, combustion air will gain heatby convection from the inner surface of the outer tube 13, both surfacesof the tube 14 and the outer surface of the tube l6.

As the" central gas tube ll'is not only covered by insulation 20 butseparated from the tube 14 by an annular air space 22 between theinsulation 20 and the inner surface of the tube 14, heat transfer to gasin the tube 11 is minimal and there is little risk of cracking of thegas with its attendant troubles.

FIG. 3 of the drawings shows a burner 24 which is generally similar inconstruction to that of the burner l of FIGS. 1 and 2 and correspondingparts are indicated by similar reference numerals. The essentialdifference is that a hole 23, or more than one hole, opens through theannular plate 19 into the annular space 22. At the rear end of theburner 24, external to the furnace, is a connection controlled by avalve (not shown), which leads from the annular space 22 to the outsideof the burner 1.

Under low-fire conditions an adequate flow of air can be maintainedthrough the air passage 12 to keep the temperature of the metal exposedto combustion products down to an acceptable level. Air entering the airbox 18 in excess of burner combustion requirements can then be bled offby suitably setting the external valve, through the hole 23, or holes23, the annular space 22, the external valve and the connection.

Under high-fire conditions the rate of flow of air through the airpassage 12 is normally determined by the combustion conditions required,for instance, to provide for stoichiometric combustion. Under high-fireconditions, therefore, the volume of air through the air passage 12would normally be that required for combustion alone ,but theconstruction of burner 24 as described may be used to enable the endwall 15 to be flush with the inner surface of the furnace wall 2 (asshown in FIG. 3), as the metal temperature there can be moderated, ifnecessary, by increasing the air flow and bleeding off through the holes23 the excess above combustion air requirements by opening the externalvalve.

However, this burner 24 having a metal end wall 15, unprotected byrefractory and flush with the inner surface of the furnace wall (asshown in. FIG. 3), can be used for operation at higher temperatures thanwould otherwise be possible; whether the burner 24 is operating athigh-fire or at low-fire, excess air canv be caused to flow through theair passages so as to moderate the temperature of the end wall 15, theexcess over combustion requirements being bled off through the holes 23and the external valve.

We claim:

1. A furnace including a self-recuperative. burner mounted in a wall ofsaid furnace and comprising a combustion chamber defining an inlet, anozzle disposed at said inlet of said combustion chamber, said burnerdefining a combustion air passage leading to said nozzle from an airinlet, a fuel passage leading to said combustion chamber and an airbleed connection including communication means leading from a pointadjacent said nozzle to outside said furnace whereby air can be bledfrom said burner before it enters said nozzle, said furnace defining aspace for products of com bustion into which an outlet of saidcombustion chamber opens and said furnace including a products passagefor conducting products of combustion to the exterior of said furnace,said air passage being disposed adjacent the. exterior'of saidbumerwhereby it is in heat-exchange relationship with said space andconstitutes a recuperator in which said products of combustion preaheatcombustion air passing along said air passage.

2. A furnace according to claim 1 wherein said combustion air passageextends at first forwards to an inlet end of said products passage andthen rearwards to said nozzle which is disposed within said wall of saidfurnace.

3. A furnace according to claim 2 wherein said combustion chamber ofsaid burner is disposed within said wall of said furnace, said outlet ofsaid combustion chamber being at an interior surface of said wall ofsaid furnace.

4. A furnace defining a space for products of combustion and includingan opening through a wall of said furnace leading from said space to theexterior of said furnace, said furnace including a self-recuperativeburner mounted in said opening in said furnace wall and comprising acombustion chamber of circular cross-section defining an inlet and anoutlet which opens into said space, a nozzle disposed at said inlet ofsaid combustion chamber, means including three tubes disposed one withinanother, each said tube being concentrically arranged with respect to anaxis of symmetry of said combustion chamber, said three tubes thusdefining inner and outer portions of a combustion air passage, anannular front wall portion connected to an inner one and an outer one ofsaid three tubes at their forward ends and disposed forwards of saidinlet of said combustion chamber, an intermediate one of said threetubes having its forward end spaced from said front wall portion todefine a passage portion connecting said inner and outer portions ofsaid combustion air passage, said inner portion of said combustion airpassage being connected to said nozzle and said outer portion of saidcombustion air passage being connected to an air inlet and disposedadjacent the-exterior of said burner, said outer one of said tubesdefining with said opening in said furnace wall a products passage forconducting products of combustion to the exterior of said furnace, saidcombustion air passage being in heat-exchange relationship with saidspace and said products passage and constituting therewith a recuperatorin which such products of combustion preheat combustion air passingalong said air passage.

5. A furnace according to claim. 4 wherein said annular front wallportion is disposed at an interior surface of said furnace wall.

1. A furnace including a self-recuperative burner mounted in a wall ofsaid furnace and comprising a combustion chamber defining an inlet, anozzle disposed at said inlet of said combustion chamber, said burnerdefining a combustion air passage leading to said nozzle from an airinlet, a fuel passage leading to said combustion chamber and an airbleed connection including communication means leading from a pointadjacent said nozzle to outside said furnace whereby air can be bledfrom said burner before it enters said nozzle, said furnace defining aspace for products of combustion into which an outlet of said combustionchamber opens and said furnace including a products passage forconducting products of combustion to the exterior of said furnace, saidair passage being disposed adjacent the exterior of said burner wherebyit is in heat-exchange relationship with said space and constitutes arecuperator in which said products of combustion pre-heat combustion airpassing along said air passage.
 2. A furnace according to claim 1wherein said combustion air passage extends at first forwards to aninlet end of said products passage and then rearwards to said nozzlewhich is disposed within said wall of said furnace.
 3. A furnaceaccording to claim 2 wherein said combustion chamber of said burner isdisposed within said wall of said furnace, said outlet of saidcombustion chamber being at an interior surface of said wall of saidfurnace.
 4. A furnace defining a space for products of combustion andincluding an opening through a wall of said furnace leading from saidspace to the exterior of said furnace, said furnace including aself-recuperative burner mounted in said opening in said furnace walland comprising a combustion chamber of circular cross-section definingan inlet and an outlet which opens into said space, a nozzle disposed atsaid inlet of said combustion chamber, means including three tubesdisposed one within another, each said tube being concentricallyarranged with respect to an axis of symmetry of said combustion chamber,said three tubes thus defining inner and outer portions of a combustionair passage, an annular front wall portion connected to an inner one andan outer one of said three tubes at their forward ends and disposedforwards of said inlet of said combustion chamber, an iNtermediate oneof said three tubes having its forward end spaced from said front wallportion to define a passage portion connecting said inner and outerportions of said combustion air passage, said inner portion of saidcombustion air passage being connected to said nozzle and said outerportion of said combustion air passage being connected to an air inletand disposed adjacent the exterior of said burner, said outer one ofsaid tubes defining with said opening in said furnace wall a productspassage for conducting products of combustion to the exterior of saidfurnace, said combustion air passage being in heat-exchange relationshipwith said space and said products passage and constituting therewith arecuperator in which such products of combustion pre-heat combustion airpassing along said air passage.
 5. A furnace according to claim 4wherein said annular front wall portion is disposed at an interiorsurface of said furnace wall.